U.S. patent number 8,325,166 [Application Number 12/481,284] was granted by the patent office on 2012-12-04 for optical device and virtual image display device including volume hologram gratings.
This patent grant is currently assigned to Sony Corporation. Invention is credited to Katsuyuki Akutsu, Hiroshi Mukawa, Satoshi Nakano.
United States Patent |
8,325,166 |
Akutsu , et al. |
December 4, 2012 |
Optical device and virtual image display device including volume
hologram gratings
Abstract
An optical device includes: a light guide plate receiving, for
each of N types of wavelength bands, a plurality of parallel light
beams with different incident angles each corresponding to view
angles, and guiding the received parallel light beams; a first and
a second volume hologram gratings of reflection type having a
diffraction configuration which includes N types of interference
fringes each corresponding to the N types of wavelength bands, and
diffracting/reflecting the parallel light beams. The optical device
satisfies for each wavelength band, a relationship of `P>L`,
where `L` represents a central diffraction wavelength in the first
and second volume hologram gratings, defined for a parallel light
beam corresponding to a central view angle, and `P` represents a
peak wavelength of the parallel light beams.
Inventors: |
Akutsu; Katsuyuki (Kanagawa,
JP), Nakano; Satoshi (Kanagawa, JP),
Mukawa; Hiroshi (Kanagawa, JP) |
Assignee: |
Sony Corporation (Tokyo,
JP)
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Family
ID: |
40999875 |
Appl.
No.: |
12/481,284 |
Filed: |
June 9, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090303212 A1 |
Dec 10, 2009 |
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Foreign Application Priority Data
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Jun 10, 2008 [JP] |
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2008-151430 |
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Current U.S.
Class: |
345/204;
345/8 |
Current CPC
Class: |
G02B
6/124 (20130101); G02B 27/0081 (20130101); G02B
27/0172 (20130101); G02B 5/203 (20130101); G02B
5/32 (20130101); G03H 1/04 (20130101); G02B
2027/0125 (20130101); G02B 2027/0116 (20130101); G02B
2027/0174 (20130101); G02B 6/00 (20130101); G02B
2027/0112 (20130101); G02B 2027/0118 (20130101); G02B
2027/011 (20130101); G03H 1/0408 (20130101) |
Current International
Class: |
G06F
3/038 (20060101); G09G 5/00 (20060101) |
Field of
Search: |
;345/204 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 536 268 |
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Jun 2005 |
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EP |
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1 619 536 |
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Jan 2006 |
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EP |
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2005-93493 |
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Oct 2005 |
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JP |
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2008-058776 |
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Mar 2006 |
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JP |
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2007-094175 |
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Apr 2007 |
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JP |
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2008-020770 |
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Jan 2008 |
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JP |
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2008-058777 |
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Mar 2008 |
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JP |
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2009-133998 |
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Jun 2009 |
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JP |
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2005-093493 |
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Oct 2005 |
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WO |
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Other References
EP Search Report for corresponding 09006129.2 dated May 12, 2011; 5
pages. cited by other .
International Search Report dated Sep. 10, 2009, for corresponding
Patent Application EP 09006129.2. cited by other.
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Primary Examiner: Eisen; Alexander
Assistant Examiner: Kiyabu; Karin
Attorney, Agent or Firm: K&L Gates LLP
Claims
The application is claimed as follows:
1. An optical device comprising: a light guide plate receiving, for
each of N types, where N is an integer of 1 or more, of wavelength
bands, a plurality of parallel light beams with different incident
angles each corresponding to view angles within a predetermined
view angle range, each of the parallel light beams traveling in
parallel, and the light guide plate guiding the received parallel
light beams according to principle of total inner reflection; a
first volume hologram grating of reflection type having a
diffraction configuration which includes N types of interference
fringes each corresponding to the N types of wavelength bands, and
diffracting and reflecting the parallel light beams which have
entered the light guide plate, so as to be reflected inside the
light guide plate according to the principle of total inner
reflection; and a second volume hologram grating of reflection type
having a diffraction configuration which includes N types of
interference fringes each corresponding to the N types of
wavelength bands, and diffracting and reflecting the parallel light
beams which have propagated inside the light guide plate according
to the principle of total inner reflection, so as to be emitted
from the light guide plate as they are in parallel, respectively,
and wherein a diffraction efficiency of the second volume hologram
grating is constant throughout the second volume hologram grating,
wherein the optical device is configured, for a wavelength band
selected from the N types of wavelength bands, to satisfy a
relationship of `P>L`, where `L` represents a central
diffraction wavelength in the first and second volume hologram
gratings, the central diffraction wavelength being defined for a
parallel light beam corresponding to a central view angle, and `P`
represents a peak wavelength of the parallel light beams which is
to enter the light guide plate.
2. A virtual image display comprising: an image forming section
displaying an image through the use of light for N types, where N
is an integer of 1 or more, of wavelength bands; a collimating
optical system converting light beams for the N types of wavelength
bands emitted from the image forming section into parallel light
beams; a light guide plate receiving, through the collimating
optical system, for each of the N types of wavelength bands, a
plurality of the parallel light beams with different incident
angles each corresponding to view angles within a predetermined
view angle range, each of the parallel light beams traveling in
parallel, and the light guide plate guiding the received parallel
light beams according to principle of total inner reflection; a
first volume hologram grating of reflection type having a
diffraction configuration which includes N kinds of interference
fringes each corresponding to the N types of wavelength bands, and
diffracting and reflecting the parallel light beams which have
entered the light guide plate, so as to be reflected inside the
light guide plate according to the principle of total inner
reflection; and a second volume hologram grating of reflection type
having a diffraction configuration which includes N types of
interference fringes each corresponding to the N types of
wavelength bands, and diffracting and reflecting the parallel light
beams which have propagated inside the light guide plate according
to the principle of total inner reflection, so as to be emitted
from the light guide plate as they are in parallel, respectively,
and wherein a diffraction efficiency of the second volume hologram
grating is constant throughout the second volume hologram grating,
wherein the virtual image display is configured, for a wavelength
band selected from the N types of wavelength bands, to satisfy a
relationship of `P>L`, where `L` represents a central
diffraction wavelength in the first and second volume hologram
gratings, the central diffraction wavelength being defined for a
parallel light beam corresponding to a central view angle, and `P`
represents a peak wavelength of the parallel light beams which is
to enter the light guide plate.
3. The optical device of claim 1, wherein the optical device is
configured to satisfy the relationship `P>L` by adjusting a
spectrum distribution of an image display element.
4. The virtual image display of claim 2, wherein the virtual image
display is configured to satisfy the relationship `P>L` by
adjusting a spectrum distribution of an image display element.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
The present application relates to Japanese Priority Patent
Application JP 2008-151430 filed in the Japanese Patent Office on
Jun. 10, 2008, the entire content of which are hereby incorporated
by reference.
BACKGROUND
The present application relates to an optical device and a virtual
image display for guiding display image light as a virtual image to
viewer's pupils through the use of a reflection type volume
hologram grating.
International Publication No. 2005/093493 pamphlet proposes a
device allowing a viewer to observe a two-dimensional image
displayed on an image display element as an enlarged virtual image
by a virtual image optical system using a reflection type volume
hologram grating. The device is a display applicable as, for
example, an HMD (Head Mounted Display). FIG. 18 illustrates a
configuration example of a virtual image display 80 proposed by
International Publication No. 2005/093493 pamphlet.
The virtual image display 80 includes an image display element 81
displaying an image, and a virtual image optical system receiving
display light displayed on the image display element 81 and then
guiding the display light to a viewer's pupil 16. The image display
element 81 is, for example, an organic EL (Electro Luminescence)
display, an inorganic EL display, a liquid crystal display (LCD) or
the like. The virtual image optical system includes a collimating
optical system 82 and a light guide plate 83 including a hologram
layer 84 arranged therein. The collimating optical system 82 is an
optical system receiving light beams emitted from pixels of the
image display element 81, and then converting the light beams into
a plurality of parallel light beams with different view angles. The
plurality of parallel light beams with different view angles
emitted from the collimating optical system 82 enters the light
guide plate 83.
FIG. 18 illustrates, as a representative of the parallel light
beams, only a parallel light beam L10 with a central view angle
which is emitted from a pixel in a central part of the image
display element 81, and then converted into a light beam with a
zero view angle (vertical to an incident surface of the light guide
plate 83) by the collimating optical system 82 to enter the light
guide plate 83.
The light guide plate 83 has a configuration in which the hologram
layer 84 is sandwiched between transparent substrates 83A and 83B.
The light guide plate 83 is a light guide plate in the shape of a
thin parallel plate including, as main surfaces, an optical surface
83a and an optical surface 83b facing the optical surface 83a. The
optical surface 83a has a light inlet 83a1 at one end thereof to
receive the parallel light beams with different view angles emitted
from the collimating optical system 82. The optical surface 83a has
a light outlet 83a2 at the other end thereof to emit light.
Protective sheets 85 and 86 for protecting the optical surfaces 83a
and 83b are arranged on the optical surfaces 83a and 83b of the
light guide plate 83, respectively. Moreover, a light-shielding
plate 87 is arranged on the protective sheet 86 arranged on the
optical surface 83b in the same position as that of the light inlet
83a1 of the light guide plate 83 to prevent a decline in light use
efficiency caused by leakage of an enlarged image displayed on the
image display element 81 and enlarged by the collimating optical
system 81 to outside of the light guide plate 83.
In the hologram layer 84, a first reflection type volume hologram
grating 84a, hereinafter described as a first grating 84a, is
formed in a position corresponding to the light inlet 83a1, and a
second reflection type volume hologram grating 84c, hereinafter
described as a second grating 84c, is formed in a position
corresponding to the light outlet 83a2. A section where the first
and second gratings 84a and 84c are not formed of the hologram
layer 84 is a non-interference-fringe-recording region 84b where
interference fringes are not recorded. In the first grating 84a,
interference fringes are recorded with uniform pitches on a
hologram surface. Moreover, in the second grating 84c, interference
fringes having different diffraction efficiency depending on their
positions are recorded. The second grating 84c has lower
diffraction efficiency in a position near the light inlet 83a1 and
higher diffraction efficiency in a position far from the light
inlet 83a1 so that light is allowed to be diffracted and reflected
a plurality of times.
The parallel light beams with different view angles entering from
the light inlet 83a1 of the light guide plate 83 enter the
above-described first grating 84a, and each of the parallel light
beams is diffracted and reflected as it is. The diffracted and
reflected parallel light beams travel while being totally reflected
between the optical surfaces 83a and 83b of the light guide plate
83 to enter the above-described second grating 84c. The light guide
plate 83 is designed to have a sufficient length in a longitudinal
direction and a thin thickness between the optical surface 83a and
the optical surface 83b so as to have such an optical path length
that numbers of times of the total reflection of the parallel light
beams with different view angles, while traveling inside the light
guide plate 83 until the parallel light beams arrive at the second
reflection grating 84c, depend on their view angles.
More specifically, among the parallel light beams entering the
light guide plate 83, a parallel light beam entering the light
guide plate 83 while being slanted toward the second grating 84c,
that is, a parallel light beam with a large incident angle is
reflected a smaller number of times than a parallel light beam
entering the light guide plate 83 while being hardly slanted toward
the second grating 84c, that is, a parallel light beam with a small
incident angle, because the parallel light beams entering the light
guide plate 83 have different view angles from one another. In
other words, the incident angles of the parallel light beams to the
first grating 84a are different from one another, so the parallel
light beams are diffracted and reflected at different diffraction
angles, thereby leading to total reflection at different angles.
Therefore, when the light guide plate 83 has a lower profile and
maintains a sufficient length in the longitudinal direction, the
numbers of times of the total reflection of the parallel light
beams are pronouncedly different from one another.
The parallel light beams with different view angles which enter the
second grating 84c are diffracted and reflected thereby to deviate
from conditions of total reflection, and then the parallel light
beams are emitted from the light outlet 83a2 of the light guide
plate 83 to enter the viewer's pupil 16.
In the virtual image display 80, when the diffraction efficiency of
the second grating 84a is changed depending on position, a pupil
diameter, that is, the virtual image viewable range of the viewer
is expanded. More specifically, for example, when the diffraction
efficiency of the second grating 84c is 40% in a position 84c1 near
the light inlet 83a1 and 70% in a position 84c2 far from the light
inlet 83a1, 40% of the parallel light beams entering the second
grating 84c for the first time is diffracted and reflected in the
position 84c1, and 60% of the parallel light beams passes through.
The parallel light beams having passing through are totally
reflected inside the light guide plate 83, and enter the position
84c2 of the second grating 84c.
The diffraction efficiency in the position 84c2 is 70%, so 60% of
the parallel light beams passes through in the first entry into the
second grating 84c, so 42% (0.6.times.0.7=0.42) of the parallel
light beams is diffracted and reflected in the position 84c2. Thus,
when the diffraction efficiency is appropriately changed depending
on the position of the second grating 84c, the light intensity
balance of light emitted from the light outlet 83a2 may be kept.
Therefore, when a region in which the interference fringes are
recorded of the second grating 84c is increased in the hologram
layer 84, the virtual image viewable range is easily expanded.
SUMMARY
However, in the virtual image display 80, as described above, among
the parallel light beams entering the light guide plate 83, the
number of times a parallel light beam entering the light guide
plate 83 while being slanted toward the second grating 84c, that
is, a parallel light beam with a large incident angle is reflected
a smaller number of times than a parallel light beam entering the
light guide plate 83 while being hardly slanted toward the second
grating 84c, that is, a parallel light beam with a small incident
angle. Therefore, the numbers of times the light beams with
different view angles are diffracted and reflected in the second
grating 84c are different from one another, so it is difficult to
keep the light intensity between the light beams with different
view angles. Referring to FIGS. 19 and 20, an issue about the light
intensity balance between the light beams with different view
angles will be described below. FIGS. 19 and 20 illustrate
simplified views of an optical system which is substantially
equivalent to a configuration of a section on the second grating
84c side of the virtual image display 80 illustrated in FIG.
18.
As illustrated in FIG. 19, a distance from a viewer's pupil
position O to the second grating 84c is S, and a light beam with a
reference view angle V is diffracted and reflected from a position
X in the second grating 84c. At this time, in a position
X.+-..theta. where light beams with a view angle .+-..theta. is
diffracted and reflected from the second grating 84c is represented
by the following expression in the case where the refractive index
of the light guide plate 83 is approximately ignored.
X.+-..theta.=X+Stan(.+-..theta.)
In this case, the view angle is an angle with respect to a normal
100 to a surface of the light guide plate 83 (a surface of the
second grating 84c). The light beam with the reference view angle V
is a light beam which enters vertically into an incident surface of
the light guide plate 83, and then is emitted vertically from an
emission surface of the light guide plate 83. That is, the
reference view angle V is 0 degrees.
A distance (X.sub.+.theta.-X.sub.-.theta.) between a position
X.sub.+.theta. and a position X.sub.-.theta. is a necessary width
of the second grating 84c in the viewer's pupil position O.
Moreover, the diffraction-reflection angle .gamma. of a parallel
light beam in a wavelength band .lamda. entering the first grating
84a with a surface pitch p at an incident angle .phi. is
represented by the following expression. In this case, the incident
angle .phi. and the diffraction-reflection angle .gamma. are angles
with respect to a normal to a surface of the first grating 84a.
Further, "n" represents the refractive index of a medium.
.gamma.=arcsin(.lamda./np-sin .phi.)
Thus, an angle at which the parallel light beams for the wavelength
band .lamda. are totally reflected inside the light guide plate 83
is changed with a change in the incident angle .phi.. Therefore, as
illustrated in FIG. 20, a number R.sub.-.theta. of times a parallel
light beam with an view angle +.theta. entering the viewer's pupil
is diffracted and reflected in the second grating 83c until the
parallel light beam arrives at the position X.sub.+.theta. and a
number Rv of times the parallel light beam with the reference view
angle V is diffracted and reflected in the second grating 83c until
the parallel light beam arrives at the position X are represented
by the following expressions in the case where the reflection
position X.sub.-.theta. of a light beam with a view angle -.theta.
is a starting point.
R.sub.+.theta.=(X.sub.+.theta.-X.sub.-.theta.)/(.lamda./np-sin(+n))))
Rv=-(X.sub.-.theta.)/(ttan(a sin(.lamda./np)))
In this case, "+.theta.n" is an angle at which the light beam with
the view angle +.theta. enters a light guide plate medium with the
refractive index n.
Thereby, for example, under the following conditions, the number Rv
of times the light beam with the reference view angle V is
diffracted and reflected is 2 in an observation position O (i.e.,
viewer's pupil position), but it is necessary for a light beam with
a view angle of +8 degrees to be diffracted and reflected four
times, and it is necessary for a light beam with a view angle of -8
degrees to be diffracted and reflected once.
Surface pitches p of the first and second grating=0.55 .mu.m
Wavelength band .lamda. (peak wavelength) of a light beam entering
the light guide plate=635 nm
Distance S to the second grating=15 mm
Thickness t of the light guide plate=1 mm
Refractive index n of the light guide plate=1.52
Peripheral view angle .+-..theta.=.+-.8 degrees
Reference view angle V=0 degrees
In International Publication No. 2005/093493 pamphlet, the
diffraction efficiency of the second grating 84c illustrated in
FIG. 18 is changed depending on position. For example, in the case
where the diffraction efficiency is changed to 40% and 70%
depending on position with reference to the reference view angle V
as a reference, in the case of a light beam with a view angle of +8
degrees, only a light intensity of 18% remains when the light beam
is diffracted and reflected for the second or subsequent times, and
most of the light intensity is lost. In other words, a light beam
with such a view angle that the light beam is diffracted and
reflected for a larger number of times by the second grating 84c
has a smaller light intensity.
As described above, in the virtual image display described in
International Publication No. 2005/093493 pamphlet, in the case
where light beams with one view angle (the reference view angle V)
are used, the virtual image viewable range may be expanded while
keeping the light intensity balance. However, when virtual images
are intended to be observed within a viewer's pupil range with
regard to parallel light beams with a plurality of view angles, it
is difficult to keep a light intensity balance between the parallel
light beams with different view angles, because the numbers of
times parallel light beams with different view angles are
diffracted and reflected in the second grating 83c are different
from one another. Therefore, unevenness in brightness in observed
virtual images occurs.
It is desirable to provide an optical device and a virtual image
display capable of favorably keeping a light intensity balance
between light beams with different view angles and capable of
observing virtual images with less unevenness in brightness.
According to an embodiment, there is provided An optical device
including: a light guide plate receiving, for each of N kinds (N is
an integer of 1 or more) of wavelength bands, a plurality of
parallel light beams with different incident angles each
corresponding to view angles within a predetermined view angle
range, each of the parallel light beams traveling in parallel, and
the light guide plate guiding the received parallel light beams
according to principle of total inner reflection; a first volume
hologram grating of reflection type having a diffraction
configuration which includes N kinds of interference fringes each
corresponding to the N kinds of wavelength bands, and diffracting
and reflecting the parallel light beams which have entered the
light guide plate, so as to be reflected inside the light guide
plate according to the principle of total inner reflection; and a
second volume hologram grating of reflection type having a
diffraction configuration which includes N kinds of interference
fringes each corresponding to the N kinds of wavelength bands, and
diffracting and reflecting the parallel light beams which have
propagated inside the light guide plate according to the principle
of total inner reflection, so as to be emitted from the light guide
plate as they are in parallel, respectively, in which the optical
device is configured, for a wavelength band selected from the N
kinds of wavelength bands, to satisfy a relationship of `P>L`,
where `L` represents a central diffraction wavelength in the first
and second volume hologram gratings, the central diffraction
wavelength being defined for a parallel light beam corresponding to
a central view angle, and `P` represents a peak wavelength of the
parallel light beams which is to enter the light guide plate.
According to an embodiment, there is provided A virtual image
display including: an image forming section displaying an image
through the use of light for N kinds (N is an integer of 1 or more)
of wavelength bands; a collimating optical system converting light
beams for the N kinds of wavelength bands emitted from the image
forming section into parallel light beams; a light guide plate
receiving, through the collimating optical system, for each of N
kinds of wavelength bands, a plurality of parallel light beams with
different incident angles each corresponding to view angles within
a predetermined view angle range, each of the parallel light beams
traveling in parallel, and the light guide plate guiding the
received parallel light beams according to principle of total inner
reflection; a first volume hologram grating of reflection type
having a diffraction configuration which includes N kinds of
interference fringes each corresponding to the N kinds of
wavelength bands, and diffracting and reflecting the parallel light
beams which have entered the light guide plate, so as to be
reflected inside the light guide plate according to the principle
of total inner reflection; and a second volume hologram grating of
reflection type having a diffraction configuration which includes N
kinds of interference fringes each corresponding to the N kinds of
wavelength bands, and diffracting and reflecting the parallel light
beams which have propagated inside the light guide plate according
to the principle of total inner reflection, so as to be emitted
from the light guide plate as they are in parallel, respectively,
in which the optical device is configured, for a wavelength band
selected from the N kinds of wavelength bands, to satisfy a
relationship of `P>L`, where `L` represents a central
diffraction wavelength in the first and second volume hologram
gratings, the central diffraction wavelength being defined for a
parallel light beam corresponding to a central view angle, and `P`
represents a peak wavelength of the parallel light beams which is
to enter the light guide plate.
In the optical device or the virtual image display according to an
embodiment, a plurality of parallel light beams enter with
different incident angles each corresponding to view angles within
a predetermined view angle range. The plurality of parallel light
beams which have entered the light guide plate are diffracted and
reflected in the first and second volume hologram gratings to be
emitted from the light guide plate. At this time, the peak
wavelength P of the parallel light beams which is to enter the
light guide plate and the central diffraction wavelength L in the
first and second volume hologram gratings satisfy a relationship of
P>L. Therefore, for each of the wavelength bands, the peak
wavelength P of the parallel light beams is brought near a central
diffraction wavelength of a light beam with a view angle which is
diffracted and reflected a large number of times in the second
volume hologram grating thereby to compensate for a decline in
light intensity of the light beam with the view angle which is
diffracted and reflected a large number of times in the second
volume hologram grating, and the light intensity balance between
light beams with different view angles is favorably maintained.
In the optical device according to an embodiment, the central
diffraction wavelength L in the first and second volume hologram
gratings and the peak wavelength P of the parallel light beams
which is to enter the light guide plate satisfy a predetermined
relationship so as to compensate for a decline in light intensity
of the light beam with a view angle which is diffracted and
reflected a large number of times in the second volume hologram
grating. Therefore, the light intensity balance between light beams
with different view angles is favorably maintained, and virtual
images with less unevenness in brightness are viewable when the
optical device is used in a virtual image display.
In the virtual image display according to an embodiment, the
central diffraction wavelength L in the first and second volume
hologram gratings and the peak wavelength P of the parallel light
beams which to be enter the light guide plate satisfy a
predetermined relationship so as to compensate for a decline in
light intensity of a light beam with a view angle diffracted and
reflected a large number of times in the second volume hologram
grating. Therefore, the light intensity balance between light beams
with different view angles is favorably maintained, and virtual
images with less unevenness in brightness are viewable.
Additional features and advantages are described in, and will be
apparent from, the following Detailed Description and the
figures.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a side view illustrating a configuration example of a
virtual image display according to an embodiment.
FIG. 2 is a side view illustrating a configuration example of a
first reflection type volume hologram grating in the virtual image
display according to an embodiment.
FIG. 3 is a side view illustrating a configuration example of a
second reflection type volume hologram grating in the virtual image
display according to an embodiment.
FIG. 4 is a side view illustrating another configuration example of
the first reflection type volume hologram grating in the virtual
image display according to an embodiment.
FIG. 5 is a side view illustrating another configuration example of
the second reflection type volume hologram grating in the virtual
image display according to an embodiment.
FIG. 6 is an illustration of a relationship between an incident
view angle and a central diffraction wavelength in the first and
the second reflection type volume hologram gratings.
FIG. 7 is an illustration of a relationship between the number of
diffraction-reflections and the intensity of emitted light in the
second reflection type volume hologram grating.
FIG. 8 is a plot illustrating an example of a spectrum distribution
of a red LED.
FIG. 9 is an illustration about a method of striking a light
intensity balance between light beams with incident view
angles.
FIG. 10 is a side view illustrating the configuration of a virtual
image display in an example of the application used for measurement
of light intensity distribution.
FIG. 11 is a plot illustrating a diffraction-reflection spectrum
distribution of a first reflection type volume hologram grating in
the example according to an embodiment.
FIG. 12 is a plot illustrating a diffraction-reflection spectrum
distribution of a second reflection type volume hologram grating in
the example according to an embodiment.
FIG. 13 is a plot illustrating a spectrum distribution of a red LED
in the example according to an embodiment.
FIG. 14 is a plot illustrating a spectrum distribution in a red
wavelength band diffracted and the reflected by the second
reflection type volume hologram grating in the example according to
an embodiment.
FIG. 15 is a plot illustrating a light intensity distribution of a
red wavelength band in a virtual image observation position in the
example according to an embodiment.
FIG. 16 is a plot illustrating a spectrum distribution of a red LED
in a comparative example.
FIG. 17 is a plot illustrating a light intensity distribution of a
red wavelength band in a virtual image observation position in the
comparative example.
FIG. 18 is a side view illustrating a configuration example of a
virtual image display in related art.
FIG. 19 is an illustration describing a diffraction-reflection
position in a second reflection type volume hologram grating of the
virtual image display in related art.
FIG. 20 is an illustration describing a relationship between the
diffraction-reflection position in the second reflection type
volume hologram grating of the virtual image display in related
art, an observation view angle and the number of
diffraction-reflections.
DETAILED DESCRIPTION
The present application will be described in detail below referring
to the accompanying drawings according to an embodiment.
FIG. 1 illustrates a configuration example of a virtual image
display 10 according to an embodiment. The virtual image display 10
includes an image display element 11 as an image forming section
displaying an image, and a virtual image optical system receiving
display light displayed on the image display element 11 to guide
the display light to a viewer's pupil 16. The image display element
11 is, for example, an organic EL display, an inorganic EL display,
a liquid crystal display or the like. The image display element 11
displays an image through the use of light for N kinds (N is an
integer of 1 or more) of wavelength bands. For example, in the case
where color display is performed, an image is displayed through the
use of light for a red wavelength band (red light), light for a
green wavelength band (green light) and light for a blue wavelength
band (blue light).
The virtual image optical system includes a collimating optical
system 12, a light guide plate 13, a first reflection type volume
hologram grating 14 and a second reflection type volume hologram
grating 15 both of which are arranged on the light guide plate
13.
The collimating optical system 12 is an optical system receiving,
for each of N kinds of wavelength bands, emitted from pixels of the
image display element 11, and then converting the light beams into
a plurality of parallel light beams with different view angles for
each of the wavelength bands. The plurality of parallel light beams
with different view angles enter the light guide plate 13.
In FIG. 1, as the plurality of parallel light beams, three parallel
light beams L10, L11 and L12 with different view angles are
illustrated. Moreover, in FIG. 1, to easily understand the state of
light rays traveling inside the light guide plate 13, the numbers
of times the light rays are reflected inside the light guide plate
13 are reduced to simplify the drawing. The parallel light beam L10
is a light beam with a central view angle which is emitted from a
pixel in a central section of the image display element 11, and is
converted into a light beam with a zero view angle (vertical to an
incident surface of the light guide plate 13) by the collimating
optical system 12 to enter the light guide plate 13. The parallel
light beam L10 corresponds to a light beam with a reference view
angle V=0.degree. illustrated in FIGS. 19 and 20. The parallel
light beam L11 is a light beam with a peripheral view angle which
is emitted from a pixel in a peripheral section of the image
display element 11, and is converted into a light beam with a
predetermined view angle (a predetermined view angle with respect
to a normal to the surface of the light guide plate 13) by the
collimating optical system 12 to enter the light guide plate 13.
The parallel light beam L11 corresponds to a light beam with a view
angle +.theta. of illustrated in FIGS. 19 and 20. The parallel
light beam L12 is a light beam with another peripheral view angle
which is emitted from a pixel in the another peripheral section of
the image display element 11, and is converted into a light beam
with another predetermined view angle (another predetermined view
angle with respect to the normal to the surface of the light guide
plate 13) by the collimating optical system 12 to enter the light
guide plate 13. The parallel light beam L12 corresponds to a light
beam with a view angle -.theta. illustrated in FIGS. 19 and 20.
The light guide plate 13 receives, for each of the N kinds of
wavelength bands, a plurality of parallel light beams with
different traveling directions through the collimating optical
system 12, and guides the received parallel light beams according
to principle of total inner reflection. The light guide plate 13 is
a light guide plate in the shape of a thin parallel plate
including, as main surfaces, an optical surface 13a and an optical
surface 13b facing the optical surface 13a. The optical surface 13a
has a light inlet 13a1 at one end thereof to receive the parallel
light beams with different view angles emitted from the collimating
optical system 12. The optical surface 13a has a light outlet 13a2
at the other end thereof to emit light. On the optical surface 13b,
the first reflection type volume hologram grating 14, hereinafter
described as the first grating 14, is arranged in a position facing
the light inlet 13a1 of the optical surface 13a, and the second
reflection type volume hologram grating 15, hereinafter described
as the second grating 15, is arranged in a position facing the
light outlet 13a2 of the optical surface 13a.
The first grating 14 diffracts and reflects the parallel light
beams for each of the wavelength bands which have entered the light
guide plate 13, so as to be reflected inside the light guide plate
13 according to the principle of total inner reflection. The second
grating 15 diffracts and reflects the parallel light beams which
have propagated inside the light guide plate 13 according to the
principle of total inner reflection, so as to be emitted from the
light guide plate 13 as they are in parallel. The first and second
gratings 14 and 15 each have a diffraction configuration which
includes N kinds of interference fringes each corresponding to the
N kinds of the wavelength bands, and interference fringes each
corresponding to the N kinds of the wavelength bands are recorded
with uniform pitches p on a hologram surface.
FIGS. 2 and 3 illustrate configuration examples of the first and
second gratings 14 and 15 each having a diffraction configuration
for three kinds (N=3) of wavelength bands, for example, red, blue
and green. As illustrated in FIG. 2, the first grating 14 is
formed, for example, by laminating three layers, that is, hologram
layers 14A, 14B and 14C. For example, interference fringes
diffracting and reflecting mainly red light are recorded in the
hologram layer 14A, and interference fringes diffracting and
reflecting mainly blue light are recorded in the hologram layer
14B, and interference fringes diffracting and reflecting mainly
green light are recorded in the hologram layer 14C. In each of the
hologram layers 14A, 14B and 14C, for example, interference fringes
with the same slant angle (slant of the interference fringes) .eta.
are recorded. The interference fringes in the hologram layer 14A,
the interference fringes in the hologram layer 14B and the
interference fringes in the hologram layer 14C are recorded with
different pitches from one another. Moreover, interference fringes
in each of the hologram layers 14A, 14B and 14C are recorded with
the same pitches irrespective of position. In other words, when the
pitches between the interference fringes recorded in the hologram
layer 14A is p, the interference fringes in the other hologram
layers 14B and 14C are recorded with pitches different from the
pitches p.
The second grating 15 has a configuration symmetrical to that of
the first grating 14. As illustrated in FIG. 3, as in the case of
the first grating 14, the second grating 15 is formed, for example,
by laminating three layers, that is, hologram layers 15A, 15B and
15C. For example, interference fringes diffracting and reflecting
mainly red light are recorded in the hologram layer 15A, and
interference fringes diffracting and reflecting mainly blue light
in the hologram layer 15B, and interference fringes diffracting and
reflecting mainly green light are recorded in the hologram layer
15C. In each of the hologram layers 15A, 15B and 15C, for example,
interference fringes with the same slant angle .eta. are recorded.
The interference fringes in the hologram layer 15A, the
interference fringes in the hologram layer 15B and the interference
fringes in the hologram layer 15C are recorded with different
pitches from one another. Moreover, interference fringes in each of
the hologram layers 15A, 15B and 15C are recorded with the same
pitches irrespective of position.
Moreover, the first and second gratings 14 and 15 have a
configuration satisfying the following condition for each of the
wavelength bands where a central diffraction wavelength defined as
a diffraction wavelength at a central view angle (the reference
view angle V) for each of the wavelength bands (for interference
fringes of each color) is L, and the peak wavelength of the
plurality of parallel light beams, for each of the wavelength
bands, entering the light guide plate 13 is P. P>L
Functions and effects by satisfying the condition will be described
in detail later.
FIGS. 4 and 5 illustrate diffraction configurations in other
configuration examples different from the configurations
illustrated in FIGS. 2 and 3. In the configuration examples,
interference fringes corresponding to N kinds of wavelength bands
are multiplexed and recorded in the same layer. In a first
reflection type volume hologram grating 24, hereinafter described
in the first grating 24, illustrated in FIG. 4, three kinds of
interference fringes diffracting and reflecting red light, green
light and blue light, that is, red light interference fringes 24R,
green light interference fringes 24G, and blue light interference
fringes 24B are multiplexed and recorded in the same layer. The
three kinds of interference fringes are recorded so that grating
pitches on a hologram surface 24S are uniform for each of the three
kinds of interference fringes, but different between the three
kinds of interference fringes. In other words, when the pitch
between the red light interference fringes 24R is p, the green
light interference fringes 24G and the blue light interference
fringe 24B are formed with different pitches from the pitch p.
Moreover, the three kinds of interference fringes are recorded, for
example, at the same slant angle .eta..
A second reflection type volume hologram grating 25, hereinafter
described the second grating 25, illustrated in FIG. 5 has a
configuration symmetric to that of the first grating 24 illustrated
in FIG. 4. As illustrated in FIG. 5, in the second grating 25, as
in the case of the first grating 24, three kinds of interference
fringes, that is, red light interference fringes 25R, green light
interference fringes 25G and blue light interference fringes 25B
are multiplexed and recorded in the same layer. The three kinds of
interference fringes are recorded so that grating pitches on a
hologram surface 25S are uniform for each of the three kinds of
interference fringes, but different between the three kinds of
interference fringes. In other words, when the pitch between the
red light interference fringes 25R is p, the green light
interference fringes 25G and the blue light interference fringes
25B are formed with different pitches from the pitch p. Moreover,
the three kinds of interference fringes are recorded, for example,
at the same slant angle .eta..
Next, the operation of the virtual image display configured in the
above-described manner will be described below.
In the virtual image display 10, the parallel light beams with
different view angles entering from the light inlet 13a1 of the
light guide plate 13 through the collimating optical system 12
enters the first grating 14, and each of the parallel light beams
is diffracted and reflected as it is. The diffracted and reflected
parallel light beams travel while being repeatedly totally
reflected between the optical surface 13a and the optical surface
13b of the light guide plate 13 to enter the second grating 15. The
light guide plate 13 is designed to have a sufficient length in a
longitudinal direction and a thin thickness between the optical
surface 13a and the optical surface 13b so as to have such an
optical path length that the numbers of times of the total
reflection of the parallel light beams with different view angles,
while traveling inside the light guide plate 13 until the parallel
light beams arrive at the second grating 15, depend on their view
angles. More specifically, among the parallel light beams entering
the light guide plate 13, the parallel light beam L11 entering at a
view angle +.theta. while being slanted toward the second grating
15, that is a parallel light beam with a large incident angle is
reflected a smaller number of times than the parallel light beam
L12 entering at a view angle -.theta. which is in an opposite
direction to the view angle +.theta..
The parallel light beams with view angles entering the second
grating 15 are diffracted and reflected thereby to deviate from
conditions of total reflection, and then the parallel light beams
are emitted from the light outlet 13a2 of the light guide plate 13
to enter a viewer's pupil 16.
In the virtual image display 10, the second grating 15 and the
first grating 14 are arranged on the optical surface 13b of the
light guide plate 13 so that interference fringes recorded in the
second grating 15 and interference fringes recorded in the first
grating 14 are 180-degree rotationally symmetric to each other in a
hologram plane. Therefore, the parallel light beams is reflected by
the second grating 15 at an angle equal to an incident angle to the
first grating 14, so a display image is displayed on the viewer's
pupil 16 with high resolution without being blurred.
Moreover, since the virtual image display 10 includes the first
grating 14 and the second grating 15 which do not work as any lens,
monochromatic eccentric aberration and diffraction chromatic
aberration may be eliminated or reduced. The first grating 14 and
the second grating 15 are arranged so that a hologram plane 14S of
the first grating 14 and a hologram plane 15S of the second grating
15 are parallel to the optical surface 13b of the light guide plate
13. However, the application is not limited thereto, and the
hologram planes 14S and 15S may be arranged so as to have a
predetermined angle with respect to the optical surface 13b.
Next, functions and effects in the case where the above-described
central diffraction wavelength L and the peak wavelength P of the
parallel light beams satisfy a predetermined relationship will be
described below. In the following description, a single wavelength
band, specifically a red wavelength band is used as an example, but
in the case where light for a plurality of wavelength bands
including other wavelength bands (a blue wavelength band, a green
wavelength band or the like) is used, when the central diffraction
wavelength L and the peak wavelength P corresponding to each of the
plurality of wavelength bands satisfy the same relationship, the
same functions and effects are obtained.
FIG. 6 illustrates a relationship between an incident or emission
view angle and a central diffraction wavelength in the first or
second grating 14 or 15. Examples of values illustrated in FIG. 6
are values under the following specifications. In addition, in the
values illustrated in FIG. 6, the view angle .theta. of a minus
value corresponds to a view angle -.theta. illustrated in FIGS. 19
and 20 in the case where the second grating 15 is used as an
example.
Specifications
Surface pitch p of first or second grating=0.535 .mu.m
Slant (slant angle) .eta. of interference fringe=64.5 degrees
Wavelength band .lamda. (peak wavelength) of light beam entering
light guide plate=635 nm
Refractive index n of light guide plate=1.52
Peripheral view angles .+-..theta.=.+-.8 degrees
The central diffraction wavelength of the first or second
reflection type volume hologram gratins 14 or 15 under the
above-described specifications are continuously shifted by the view
angle as illustrated in FIG. 6, because Bragg conditions are
changed depending on the incident angle of the parallel light beam.
In other words, as the view angle increases, the central
diffraction wavelength increases, and it is found out that the
central diffraction wavelengths at a view angle +.theta., the
central view angle V (=0 degrees) and a view angle -.theta. are 660
nm, 635 nm and 605 nm, respectively.
Now, as described above referring to FIG. 20, the view angle of a
parallel light beam which is totally reflected inside the second
grating 15 a larger number of times causes an increase in the
number of times the light beam is diffracted and reflected by the
second grating 15, thereby an image is dark when a viewer observes
the image. For example, when the diffraction efficiency of the
second grating 15 is 30%, and the intensity of a light beam
entering the second grating 15 for the first time is 100%, the
intensity of the light beam diffracted and reflected for the first
time to be emitted is 30%, and the intensity of the emitted light
beam diffracted and reflected for the second time is 21% because
30% of the intensity (70%) of the light beam not diffracted and
reflected for the first time is diffracted, and in the same manner,
the intensity of the light beam diffracted and reflected for the
third time to be emitted is 14.7%, and the intensity of the light
beam diffracted and reflected for the fourth time to be emitted is
10.29%. Thus, the intensity of a light beam with a view angle which
is viewable by being diffracted and reflected for the fourth time
is about 1/3 of the intensity of a light beam with a view angle
which is viewable by being diffracted and reflected for the first
time. Such a decline in light intensity occurs even in the case
where the diffraction efficiency of the second grating 15 is
changed.
Moreover, as illustrated in FIG. 7, it is found out that even in
the case where the diffraction efficiency is changed within a range
of 10% to 40%, the larger the number of times light with a view
angle is diffracted and reflected, the more the intensity of the
light with the view angle is reduced in principle. FIG. 7
illustrates a relationship between the number of times light is
internally diffracted and reflected and the intensity of emitted
light in the second grating 15 in the case where the diffraction
efficiency is changed.
On the other hand, for example, in the case where a red LED with a
spectrum distribution having a peak around 650 nm as illustrated in
FIG. 8 is used as a light source of the image display element 11,
the spectrum distribution of light with each view angle which is
diffracted and reflected from the second grating 15 is represented
by the product of the diffraction efficiency distributions of the
first and second gratings 14 and 15 in which the central
diffraction wavelength is shifted by the spectrum distribution of
the LED and Bragg conditions.
In the embodiment, to solve an issue of the above-described decline
in light intensity, as illustrated in FIG. 9, the peak wavelength
of the LED is brought near the central diffraction wavelength of a
light beam with a view angle which is diffracted and reflected a
large number of times, for example, 3 or 4 times thereby to
compensate for a decline in light intensity of the light beam with
a view angle which is diffracted and reflected a large number of
times and to increase the intensity of the light beam with the view
angle. Therefore, the light intensity balance between light beams
with different view angles is struck. In other words, it means that
when a center wavelength (a central diffraction wavelength at a
view angle of 0 degrees) for a wavelength band diffracted and
reflected by the first and second gratings 14 and 15 is L (635 nm),
and the peak wavelength of the parallel light beams having the
spectrum distribution of the LED entering the light guide plate 13
for the wavelength band is P (648 nm), a relationship between them
is represented by the following relationship. P>L
Next, a specific example of the virtual image display 10 according
to the embodiment will be described below.
FIG. 10 illustrates the configuration of the virtual image display
10 in the example. In the example, the virtual image display 10 was
formed so that the thickness of the first grating 14 was 7 .mu.m,
and the thickness of the second grating 15 was 5 .mu.m, and the
surface pitches p of the first and second gratings 14 and 15 was
0.531 .mu.m, and the slant angle .eta. of the interference fringe
was 64.5 degrees, and .DELTA.n was 0.05. In this case, ".DELTA.n"
represents a modulated width of the refractive index of each of the
first and second gratings 14 and 15 diffracting and reflecting
light beams by the periodical modulation of the refractive index in
a medium. The first and second gratings 14 and 15 were arranged on
the light guide plate 13 with a thickness of 1 mm so as to have a
space of 30 mm therebetween, and the parallel light beams emitted
from the image display element 11 and collimated at a view angle of
.+-.8 degrees by the collimating optical system 12 was allowed to
enter the first grating 14, and a virtual image was observed by the
CCD camera 17 at the viewer's pupil position O.
The wavelength band diffracted and reflected by the first and
second gratings 14 and 15 in the example was 585 nm to 670 nm in a
range of the view angle (i.e., incident angle) of .+-.8 degrees as
illustrated in FIGS. 11 and 12, and the central wavelength was
substantially equal to a diffraction wavelength of 630 nm at the
central view angle of 0 degrees. FIG. 11 illustrates the
diffraction reflection spectrum of the first grating 14, and FIG.
12 illustrates the diffraction reflection spectrum of the second
grating 15.
FIG. 13 illustrates the spectrum distribution of a light source
(the red LED) illuminating the image display element 11 used in the
example. The diffraction efficiency distributions at each view
angle of the first and second gratings 14 and 15 are as illustrated
in FIGS. 11 and 12, and in the case where a light source
illustrated in FIG. 13 is used, the LED spectrums of light beams,
diffracted and reflected by the second grating 15, at the view
angle of .+-.8 degrees and the central view angle are as
illustrated in FIG. 14.
A result obtained by measuring a light intensity distribution in a
horizontal direction of a virtual image plane observed in the
viewer's pupil position O in the example by the CCD camera 17 is
illustrated in FIG. 15. The peak wavelength of the red LED at this
time was 645 nm, and the peak wavelength compensated for a decline
in the intensity of a light beam with a view angle of +8 degrees by
diffracting and reflecting the light beam a plurality of times.
Then, a relationship between the central wavelength L (630 nm) for
a wavelength band diffracted and reflected by the first and second
gratings 14 and 15 and the peak wavelength P=645 nm of the parallel
light beams by the red LED in the example, which entered the light
guide plate 13 for the wavelength band satisfied the following
relationship. P>L
Next, as a comparative example, measurement was performed in the
case where a red LED having a spectrum distribution illustrated in
FIG. 16 was used as a light source illuminating the image display
element 11. A result obtained by measuring the light intensity
distribution in a horizontal direction of a virtual image plane
observed in the viewer's pupil position O is illustrated in FIG.
17. In the comparative example, the peak wavelength of the red LED
was 630 nm, and a relationship, between the central wavelength
L=635 nm for a wavelength band where light beams were diffracted
and reflected by the first and second gratings 14 and 15 and the
peak wavelength P=630 nm of the plurality of parallel light beams
by the red LED entering the light guide plate 13 for the wavelength
band, did not satisfy the above relationship.
It was evident from a comparison between the results obtained by
measuring the light intensity distributions illustrated in FIGS. 15
and 17 that in the case of the comparative example in FIG. 17 in
which the relationship did not satisfy P>L, the intensity of a
light beam with a view angle which was reflected a smaller number
of times is higher, and the intensity of a light beam with a view
angle which was reflected a larger number of times was lower, so a
virtual image displayed thereby was not appropriate as an observed
image. On the other hand, in the case of the example in FIG. 15 in
which the relationship satisfied the above relationship, the
luminance at the central view angle in the observed virtual image
was the highest, and as the view angle increased in positive or
negative directions, the luminance gradually declined. This was a
natural state as an observed image, and the light intensity balance
between light beams with different view angles was struck.
In the above-described example, the case where the relationship
P>L is satisfied by changing the spectrum distribution of the
light source of the image display element 11 is described. However,
the relationship P>L may be satisfied by changing the
diffraction configurations of the first and second gratings 14 and
15.
As described above, in the virtual image display 10 according to
the embodiment, the central diffraction wavelength L in the first
and second gratings 14 and 15 and the peak wavelength P of the
parallel light beams entering the light guide plate 13 satisfy a
predetermined relationship so as to compensate for a decline in
intensity of the light beam with a view angle which is diffracted
and reflected a large number of times in the second grating 15, so
the light intensity balance between light beams with different view
angles is favorably maintained, and virtual images with less
unevenness in brightness may be observed.
Other Embodiment
The present application is not limited to the above-described
embodiment, and may be variously modified.
For example, the present application is applicable to a
configuration example of a virtual image display 80 illustrated in
FIG. 18, as in the case of the configuration example illustrated in
FIG. 1. More specifically, it is only necessary for first and
second gratings 84a and 84c in the virtual image display 80 to be
configured so that a central diffraction wavelength L at a central
view angle satisfies the following relationship with a peak
wavelength P of parallel light beams which is to enter a light
guide plate 83. P>L
Moreover, the present application is applicable to apparatuses
displaying an enlarged virtual image in substantially the same
principle as that in the virtual image display illustrated in FIG.
1 or FIG. 18 through the use of a reflection type volume hologram
grating.
It should be understood that various changes and modifications to
the presently preferred embodiments described herein will be
apparent to those skilled in the art. Such changes and
modifications can be made without departing from the spirit and
scope and without diminishing its intended advantages. It is
therefore intended that such changes and modifications be covered
by the appended claims.
* * * * *